Syria and the United States are both attacking Islamic State militants who have displaced thousands of civilians in Syria and northern Iraq. But, U.S. airstrikes in Iraq could push those forces back across the border and again into the fight against Syrian President Bashar al-Assad.

While the fighters of the Islamic State have been sweeping through Iraq in recent weeks, Syrian troops have kept up their fight against both it and the opposition rebels - regaining ground near the Damascus airport while bombing militant positions near the border. And civilians continue to flee the fighting, says Human Rights Watch's Sarah Margon.

"We are looking at a very scaled-up, brutal situation not only in Syria, but also in Iraq where certainly the borders are increasingly blurred," she said.

But facing a counter-attack by the Kurdish Peshmerga, and unable to counter U.S. airstrikes in Iraq, U.S. Institute of Peace analyst Steve Heydemann says Islamic State fighters may also be more apt to refocus on Syria,

"It would be even more likely to become their next phase of activity if, in fact, the American operations in Iraq do slow their advances there and give them incentives to turn their attentions away from their more-difficult front to a front in which the Americans are not present," he said.

And he says with weapons and territory captured in Iraq, the Islamic State militants may now also be in a stronger position to confront Syrian government forces.

"It may be that they have reached a point where they can make a bit of a tactical pivot," Heydemann said. "Having secured a solid base, they can perhaps turn their attention more fully to the regime."

Even though the U.S. is launching air strikes against the same group that Syrian government forces are fighting - that does not give Washington common cause with Damascus, says Deputy U.S. State Department Spokeswoman Marie Harf.

"While we may be looking at some of the same targets, I think the fact that, or targets from the same group, the fact that the Assad regime has allowed ISIS to flourish and grow in the way it has is really one of the main reasons they have grown so strong," she said.

That early alliance of convenience with ISIS against more moderate opponents is something Heydemann says Assad may find to have been a miscalculation.

"One of the big questions is whether in creating the conditions that permitted the rise of ISIS, including some direct measures like the release of militants from Syrian prisons who then joined ISIS, the regime will find that the backlash creates a much bigger problem than it anticipated," he said.

A problem that, at least for now, Syrian forces are getting help with - in the international drive against the Islamic State.